The structure of PSI-LHCI from Cyanidium caldarium provides evolutionary insights into conservation and diversity of red-lineage LHCs.

Light-harvesting complexes (LHCs) are diversified among photosynthetic organisms, and the structure of the photosystem I-LHC (PSI-LHCI) supercomplex has been shown to be variable depending on the species of organisms. However, the structural and evolutionary correlations of red-lineage LHCs are unknown. Here, we determined a 1.92-Å resolution cryoelectron microscopic structure of a PSI-LHCI supercomplex isolated from the red alga Cyanidium caldarium RK-1 (NIES-2137), which is an important taxon in the Cyanidiophyceae. We subsequently investigated the correlations of PSI-LHCIs from different organisms through structural comparisons and phylogenetic analysis. The PSI-LHCI structure obtained shows five LHCI subunits surrounding a PSI-monomer core. The five LHCIs are composed of two Lhcr1s, two Lhcr2s, and one Lhcr3. Phylogenetic analysis of LHCs bound to PSI in the red-lineage algae showed clear orthology of LHCs between C. caldarium and Cyanidioschyzon merolae, whereas no orthologous relationships were found between C. caldarium Lhcr1-3 and LHCs in other red-lineage PSI-LHCI structures. These findings provide evolutionary insights into conservation and diversity of red-lineage LHCs associated with PSI.

Sensing chemical-induced DNA damage using CRISPR/Cas9-mediated gene-deletion yeast-reporter strains.

Microorganism-based genotoxicity assessments are vital for evaluating potential chemical-induced DNA damage. In this study, we developed both chromosomally integrated and single-copy plasmid-based reporter assays in budding yeast using a RNR3 promoter-driven luciferase gene. These assays were designed to compare the response to genotoxic chemicals with a pre-established multicopy plasmid-based assay. Despite exhibiting the lowest luciferase activity, the chromosomally integrated reporter assay showed the highest fold induction (i.e., the ratio of luciferase activity in the presence and absence of the chemical) compared with the established plasmid-based assay. Using CRISPR/Cas9 technology, we generated mutants with single- or double-gene deletions, affecting major DNA repair pathways or cell permeability. This enabled us to evaluate reporter gene responses to genotoxicants in a single-copy plasmid-based assay. Elevated background activities were observed in several mutants, such as mag1Δ cells, even without exposure to chemicals. However, substantial luciferase induction was detected in single-deletion mutants following exposure to specific chemicals, including mag1Δ, mms2Δ, and rad59Δ cells treated with methyl methanesulfonate; rad59Δ cells exposed to camptothecin; and mms2Δ and rad10Δ cells treated with mitomycin C (MMC) and cisplatin (CDDP). Notably, mms2Δ/rad10Δ cells treated with MMC or CDDP exhibited significantly enhanced luciferase induction compared with the parent single-deletion mutants, suggesting that postreplication and for nucleotide excision repair processes predominantly contribute to repairing DNA crosslinks. Overall, our findings demonstrate the utility of yeast-based reporter assays employing strains with multiple-deletion mutations in DNA repair genes. These assays serve as valuable tools for investigating DNA repair mechanisms and assessing chemical-induced DNA damage. KEY POINTS: • Responses to genotoxic chemicals were investigated in three types of reporter yeast. • Yeast strains with single- and double-deletions of DNA repair genes were tested. • Two DNA repair pathways predominantly contributed to DNA crosslink repair in yeast.

Structural basis of the protochromic green/red photocycle of the chromatic acclimation sensor RcaE.

Cyanobacteriochromes (CBCRs) are bilin-binding photosensors of the phytochrome superfamily that show remarkable spectral diversity. The green/red CBCR subfamily is important for regulating chromatic acclimation of photosynthetic antenna in cyanobacteria and is applied for optogenetic control of gene expression in synthetic biology. It is suggested that the absorption change of this subfamily is caused by the bilin C15-Z/C15-E photoisomerization and a subsequent change in the bilin protonation state. However, structural information and direct evidence of the bilin protonation state are lacking. Here, we report a high-resolution (1.63Å) crystal structure of the bilin-binding domain of the chromatic acclimation sensor RcaE in the red-absorbing photoproduct state. The bilin is buried within a "bucket" consisting of hydrophobic residues, in which the bilin configuration/conformation is C5-Z,syn/C10-Z,syn/C15-E,syn with the A- through C-rings coplanar and the D-ring tilted. Three pyrrole nitrogens of the A- through C-rings are covered in the α-face with a hydrophobic lid of Leu249 influencing the bilin pKa, whereas they are directly hydrogen bonded in the ß-face with the carboxyl group of Glu217. Glu217 is further connected to a cluster of waters forming a hole in the bucket, which are in exchange with solvent waters in molecular dynamics simulation. We propose that the "leaky bucket" structure functions as a proton exit/influx pathway upon photoconversion. NMR analysis demonstrated that the four pyrrole nitrogen atoms are indeed fully protonated in the red-absorbing state, but one of them, most likely the B-ring nitrogen, is deprotonated in the green-absorbing state. These findings deepen our understanding of the diverse spectral tuning mechanisms present in CBCRs.

A hybrid type of chromatic acclimation regulated by the dual green/red photosensory systems in cyanobacteria.

Cyanobacteria are phototrophic bacteria that perform oxygenic photosynthesis. They use a supermolecular light-harvesting antenna complex, the phycobilisome (PBS), to capture and transfer light energy to photosynthetic reaction centers. Certain cyanobacteria alter the absorption maxima and/or overall structure of their PBSs in response to the ambient light wavelength-a process called chromatic acclimation (CA). One of the most well-known CA types is the response to green and red light, which is controlled by either the RcaEFC or CcaSR photosensory system. Here, we characterized a hybrid type of CA in the cyanobacterium Pleurocapsa sp. Pasteur Culture Collection (PCC) 7319 that uses both RcaEFC and CcaSR systems. In vivo spectroscopy suggested that strain PCC 7319 alters the relative composition of green-absorbing phycoerythrin and red-absorbing phycocyanin in the PBS. RNA sequencing and promoter motif analyses suggested that the RcaEFC system induces a gene operon for phycocyanin under red light, whereas the CcaSR system induces a rod-membrane linker gene under green light. Induction of the phycoerythrin genes under green light may be regulated through a yet unidentified photosensory system called the Cgi system. Spectroscopy analyses of the isolated PBSs suggested that hemidiscoidal and rod-shaped PBSs enriched with phycoerythrin were produced under green light, whereas only hemidiscoidal PBSs enriched with phycocyanin were produced under red light. PCC 7319 uses the RcaEFC and CcaSR systems to regulate absorption of green or red light (CA3) and the amount of rod-shaped PBSs (CA1), respectively. Cyanobacteria can thus flexibly combine diverse CA types to acclimate to different light environments.

Microbiome analyses of 12 psyllid species of the family Psyllidae identified various bacteria including Fukatsuia and Serratia symbiotica, known as secondary symbionts of aphids.

BACKGROUND: Psyllids (Hemiptera: Psylloidea) comprise a group of plant sap-sucking insects that includes important agricultural pests. They have close associations not only with plant pathogens, but also with various microbes, including obligate mutualists and facultative symbionts. Recent studies are revealing that interactions among such bacterial populations are important for psyllid biology and host plant pathology. In the present study, to obtain further insight into the ecological and evolutionary behaviors of bacteria in Psylloidea, we analyzed the microbiomes of 12 psyllid species belonging to the family Psyllidae (11 from Psyllinae and one from Macrocorsinae), using high-throughput amplicon sequencing of the 16S rRNA gene. RESULTS: The analysis showed that all 12 psyllids have the primary symbiont, Candidatus Carsonella ruddii (Gammaproteobacteria: Oceanospirillales), and at least one secondary symbiont. The majority of the secondary symbionts were gammaproteobacteria, especially those of the family Enterobacteriaceae (order: Enterobacteriales). Among them, symbionts belonging to "endosymbionts3", which is a genus-level monophyletic group assigned by the SILVA rRNA database, were the most prevalent and were found in 9 of 11 Psyllinae species. Ca. Fukatsuia symbiotica and Serratia symbiotica, which were recognized only as secondary symbionts of aphids, were also identified. In addition to other Enterobacteriaceae bacteria, including Arsenophonus, Sodalis, and "endosymbionts2", which is another genus-level clade, Pseudomonas (Pseudomonadales: Pseudomonadaceae) and Diplorickettsia (Diplorickettsiales: Diplorickettsiaceae) were identified. Regarding Alphaproteobacteria, the potential plant pathogen Ca. Liberibacter europaeus (Rhizobiales: Rhizobiaceae) was detected for the first time in Anomoneura mori (Psyllinae), a mulberry pest. Wolbachia (Rickettsiales: Anaplasmataceae) and Rickettsia (Rickettsiales: Rickettsiaceae), plausible host reproduction manipulators that are potential tools to control pest insects, were also detected. CONCLUSIONS: The present study identified various bacterial symbionts including previously unexpected lineages in psyllids, suggesting considerable interspecific transfer of arthropod symbionts. The findings provide deeper insights into the evolution of interactions among insects, bacteria, and plants, which may be exploited to facilitate the control of pest psyllids in the future.

Algorisms used for in silico finishing of bacterial genomes based on short-read assemblage implemented in GenoFinisher, AceFileViewer, and ShortReadManager.

In these days, for bacterial genome sequence determination, ultralong reads with homopolymeric troubles are used in combinations with short reads, resulting in genomic sequences with possible incorrect uniformity of repeat sequences. We have been determining complete bacterial genomic sequences based on NGS short reads and Newbler assemblage by utilizing functions implemented in 3 software GenoFinisher, AceFileViewer, and ShortReadManager without conducting additional experiments for gap closing, proving the concept that NGS short reads enclose enough information to determine complete genome sequences. Although some manual in silico tasks are to be conducted, they will ultimately be solved in a single pipeline. In this review, we describe the tools and implemented ideas that have enabled complete sequence determination solely based on short reads, which would be useful for establishing the basis for the future development of a short-read-based assembler that enables complete and accurate genome sequence determination at a lower cost.

Pressurized Liquid Extraction of a Phycocyanobilin Chromophore and Its Reconstitution with a Cyanobacteriochrome Photosensor for Efficient Isotopic Labeling.

Linear tetrapyrrole compounds (bilins) are chromophores of the phytochrome and cyanobacteriochrome classes of photosensors and light-harvesting phycobiliproteins. Various spectroscopic techniques, such as resonance Raman, Fourier transform-infrared and nuclear magnetic resonance, have been used to elucidate the structures underlying their remarkable spectral diversity, in which the signals are experimentally assigned to specific structures using isotopically labeled bilin. However, current methods for isotopic labeling of bilins require specialized expertise, time-consuming procedures and/or expensive reagents. To address these shortcomings, we established a method for pressurized liquid extraction of phycocyanobilin (PCB) from the phycobiliprotein powder Lina Blue and also the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). PCB was efficiently cleaved in ethanol with three extractions (5 min each) under nitrogen at 125�C and 100 bars. A prewash at 75�C was effective for removing cellular pigments of Synechocystis without PCB cleavage. Liquid chromatography and mass spectrometry suggested that PCB was cleaved in the C3-E (majority) and C3-Z (partial) configurations. 15N- and 13C/15N-labeled PCBs were prepared from Synechocystis cells grown with NaH13CO3 and/or Na15NO3, the concentrations of which were optimized based on cell growth and pigmentation. Extracted PCB was reconstituted with a recombinant apoprotein of the cyanobacteriochrome-class photosensor RcaE. Yield of the photoactive holoprotein was improved by optimization of the expression conditions and cell disruption in the presence of Tween 20. Our method can be applied for the isotopic labeling of other PCB-binding proteins and for the commercial production of non-labeled PCB for food, cosmetic and medical applications.

Protochromic absorption changes in the two-cysteine photocycle of a blue/orange cyanobacteriochrome.

Cyanobacteriochromes (CBCRs) are phytochrome-related photosensors with diverse spectral sensitivities spanning the entire visible spectrum. They covalently bind bilin chromophores via conserved cysteine residues and undergo 15Z/15E bilin photoisomerization upon light illumination. CBCR subfamilies absorbing violet-blue light use an additional cysteine residue to form a second bilin-thiol adduct in a two-Cys photocycle. However, the process of second thiol adduct formation is incompletely understood, especially the involvement of the bilin protonation state. Here, we focused on the Oscil6304_2705 protein from the cyanobacterium Oscillatoria acuminata PCC 6304, which photoconverts between a blue-absorbing 15Z state ( 15Z Pb) and orange-absorbing 15E state ( 15E Po). pH titration analysis revealed that 15Z Pb was stable over a wide pH range, suggesting that bilin protonation is stabilized by a second thiol adduct. As revealed by resonance Raman spectroscopy, 15E Po harbored protonated bilin at both acidic and neutral pH, but readily converted to a deprotonated green-absorbing 15Z state ( 15Z Pg) at alkaline pH. Site-directed mutagenesis revealed that the conserved Asp-71 and His-102 residues are required for second thiol adduct formation in 15Z Pb and bilin protonation in 15E Po, respectively. An Oscil6304_2705 variant lacking the second cysteine residue, Cys-73, photoconverted between deprotonated 15Z Pg and protonated 15E Pr, similarly to the protochromic photocycle of the green/red CBCR subfamily. Time-resolved spectroscopy revealed 15Z Pg formation as an intermediate in the 15E Pr-to- 15Z Pg conversion with a significant solvent-isotope effect, suggesting the sequential occurrence of 15EP-to-15Z photoisomerization, deprotonation, and second thiol adduct formation. Our findings uncover the details of protochromic absorption changes underlying the two-Cys photocycle of violet-blue-absorbing CBCR subfamilies.

16S rRNA Sequencing Detected Profftella, Liberibacter, Wolbachia, and Diplorickettsia from Relatives of the Asian Citrus Psyllid.

The Asian citrus psyllid Diaphorina citri (Hemiptera: Psylloidea) is a serious pest of citrus species worldwide because it transmits Candidatus Liberibacter spp. (Alphaproteobacteria: Rhizobiales), the causative agents of the incurable citrus disease, huanglongbing or greening disease. Diaphorina citri possesses a specialized organ called a bacteriome, which harbors vertically transmitted intracellular mutualists, Ca. Carsonella ruddii (Gammaproteobacteria: Oceanospirillales) and Ca. Profftella armatura (Gammaproteobacteria: Betaproteobacteriales). Whereas Carsonella is a typical nutritional symbiont, Profftella is an unprecedented type of toxin-producing defensive symbiont, unusually sharing organelle-like features with nutritional symbionts. Additionally, many D. citri strains are infected with Wolbachia, which manipulate reproduction in various arthropod hosts. In the present study, in an effort to obtain insights into the evolution of symbioses between Diaphorina and bacteria, microbiomes of psyllids closely related to D. citri were investigated. Bacterial populations of Diaphorina cf. continua and Diaphorina lycii were analyzed using Illumina sequencing of 16S rRNA gene amplicons and compared with data obtained from D. citri. The analysis revealed that all three Diaphorina spp. harbor Profftella as well as Carsonella lineages, implying that Profftella is widespread within the genus Diaphorina. Moreover, the analysis identified Ca. Liberibacter europaeus and Diplorickettsia sp. (Gammaproteobacteria: Diplorickettsiales) in D. cf. continua, and a total of four Wolbachia (Alphaproteobacteria: Rickettsiales) lineages in the three psyllid species. These results provide deeper insights into the interactions among insects, bacteria, and plants, which would eventually help to better manage horticulture.

Characterization of a novel class of glyoxylate reductase belonging to the ß-hydroxyacid dehydrogenase family in Acetobacter aceti.

Enzymes related to ß-hydroxyacid dehydrogenases/3-hydroxyisobutyrate dehydrogenases are ubiquitous, but most of them have not been characterized. An uncharacterized protein with moderate sequence similarities to Gluconobacter oxydans succinic semialdehyde reductase and plant glyoxylate reductases/succinic semialdehyde reductases was found in the genome of Acetobacter aceti JCM20276. The corresponding gene was cloned and expressed in Escherichia coli. The gene product was purified and identified as a glyoxylate reductase that exclusively catalyzed the NAD(P)H-dependent reduction of glyoxylate to glycolate. The strict substrate specificity of this enzyme to glyoxylate, the diverged sequence motifs for its binding sites with cofactors and substrates, and its phylogenetic relationship to homologous enzymes suggested that this enzyme represents a novel class of enzymes in the ß-hydroxyacid dehydrogenase family. This study may provide an important clue to clarify the metabolism of glyoxylate in bacteria. Abbreviations: GR: glyoxylate reductase; GRHPR: glyoxylate reductase/hydroxypyruvate reductase; HIBADH: 3-hydroxyisobutyrate dehydrogenase; SSA: succinic semialdehyde; SSAR: succinic semialdehyde reductase.

Acidophilic green algal genome provides insights into adaptation to an acidic environment.

Some microalgae are adapted to extremely acidic environments in which toxic metals are present at high levels. However, little is known about how acidophilic algae evolved from their respective neutrophilic ancestors by adapting to particular acidic environments. To gain insights into this issue, we determined the draft genome sequence of the acidophilic green alga Chlamydomonas eustigma and performed comparative genome and transcriptome analyses between Ceustigma and its neutrophilic relative Chlamydomonas reinhardtii The results revealed the following features in Ceustigma that probably contributed to the adaptation to an acidic environment. Genes encoding heat-shock proteins and plasma membrane H+-ATPase are highly expressed in Ceustigma This species has also lost fermentation pathways that acidify the cytosol and has acquired an energy shuttle and buffering system and arsenic detoxification genes through horizontal gene transfer. Moreover, the arsenic detoxification genes have been multiplied in the genome. These features have also been found in other acidophilic green and red algae, suggesting the existence of common mechanisms in the adaptation to acidic environments.

Unusual features in the photosynthetic machinery of Halorhodospira halochloris DSM 1059 revealed by complete genome sequencing.

Halorhodospira halochloris is an anaerobic, halophilic, purple photosynthetic bacterium belonging to γ-Proteobacteria. H. halochloris is also characteristic as a thermophilic phototrophic isolate producing bacteriochlorophyll (BChl) b. Here, we report the complete genome sequence of H. halochloris DSM 1059. The genetic arrangement for this bacterium's photosynthetic apparatus is of particular interest; its genome contains two sets of puf operons encoding the reaction center and core light-harvesting 1 (LH1) complexes having almost identical nucleotide sequences (e.g., 98.8-99.9% of nucleotide identities between two sets of pufLM genes, but 100% of deduced amino acid sequence identities). This duplication of photosynthetic genes may provide a glimpse at natural selection in action. The ß-polypeptides of the LH1 complex in purple bacteria usually contain two histidine residues to bind BChl a; however, those of H. halochloris were revealed to have four histidine residues, indicating unusual pigment organization in the LH1 complex of this species. Like in other BChl b-producing phototrophs, the genome of H. halochloris lacks the divinyl reductase genes bciA and bciB. The phylogeny of chlorophyllide a oxidoreductase, which catalyzes committed steps in the synthesis of BChl a and BChl b, indicates that evolution toward BChl b production is convergent. Geranylgeranyl reductase (BchP) of H. halochloris has an insertion region in its primary structure, which could be important for its unusual sequential reduction reactions.

Development and evaluation of yeast-based GFP and luciferase reporter assays for chemical-induced genotoxicity and oxidative damage.

We aimed to develop the bioassays for genotixicity and/or oxidative damage using the recombinant yeast. A genotoxicity assay was developed using recombinant Saccharomyces cerevisiae strain BY4741 with a green fluorescent protein (GFP) reporter plasmid, driven by the DNA damage-responsive RNR3 promoter. Enhanced fluorescence induction was observed in DNA repair-deficient strains treated with methyl methanesulfonate, but not with hydrogen peroxide. A GFP reporter yeast strain driven by the oxidative stress-responsive TRX2 promoter was newly developed to assess oxidative damage, but fluorescence was poorly induced by oxidants. In place of GFP, yeast strains with luciferase gene reporter plasmids (luc2 and luc2CP, encoding stable and unstable luciferase, respectively) were prepared. Transient induction of luciferase activity was clearly detected only in a TRX2 promoter-driven luc2CP reporter strain within 90 min of oxidant exposure. However, luciferase was strongly induced by hydroxyurea in the RNR3 promoter-driven luc2 and GFP reporter strains over 8 h after the exposure, suggesting that the RNR3 promoter is continuously upregulated by DNA damage, whereas the TRX2 promoter is transiently activated by oxidative agents. Luciferase activity levels were also increased in a TRX2-promoter-driven luc2CP reporter strain treated with tert-butyl hydroperoxide and menadione and weakly induced with diamide and diethyl maleate. Weakly enhanced luciferase activity induction was detected in the sod1Δ, sod2Δ, and rad27Δ strains treated with hydrogen peroxide compared with that in the wild-type strain. In conclusion, tests using GFP and stable luciferase reporters are useful for genotoxicity, and oxidative damage can be clearly detected by assay with an unstable luciferase reporter.

A high-molecular-weight, alkaline, and thermostable ß-1,4-xylanase of a subseafloor Microcella alkaliphila.

An endo ß-1,4-xylanase (XynE15) from a culture broth of a deep subseafloor microorganism, Microcella alkaliphila JAM-AC0309, was purified to homogeneity. The molecular mass of XynE15 was approximately 150 kDa as judged by SDS-PAGE. The optimal pH and temperature for hydrolysis of xylan were pH 8 and 65 °C. The enzyme was stable to incubation for 30 min at up to 75 °C, and the half-life at 50 °C was 48 h. XynE15 hydrolyzed arabinoxylan, oat spelt xylan, and birchwood xylan well, but not avicel, carboxymethylcellulose, or arabinan. Xylooligosaccharides were hydrolyzed to mainly xylobiose from higher than xylotetraose. The genome sequencing analysis of strain JAM-AC03039 revealed that XynE15 was composed of 1,319 amino acids with one catalytic domain and three carbohydrate-binding domains belonging to glycoside hydrolase (GH) family 10 and carbohydrate-binding module (CBM) family 4, respectively.

Green/red cyanobacteriochromes regulate complementary chromatic acclimation via a protochromic photocycle.

Cyanobacteriochromes (CBCRs) are cyanobacterial members of the phytochrome superfamily of photosensors. Like phytochromes, CBCRs convert between two photostates by photoisomerization of a covalently bound linear tetrapyrrole (bilin) chromophore. Although phytochromes are red/far-red sensors, CBCRs exhibit diverse photocycles spanning the visible spectrum and the near-UV (330-680 nm). Two CBCR subfamilies detect near-UV to blue light (330-450 nm) via a "two-Cys photocycle" that couples bilin 15Z/15E photoisomerization with formation or elimination of a second bilin-cysteine adduct. On the other hand, mechanisms for tuning the absorption between the green and red regions of the spectrum have not been elucidated as of yet. CcaS and RcaE are members of a CBCR subfamily that regulates complementary chromatic acclimation, in which cyanobacteria optimize light-harvesting antennae in response to green or red ambient light. CcaS has been shown to undergo a green/red photocycle: reversible photoconversion between a green-absorbing 15Z state ((15Z)P(g)) and a red-absorbing 15E state ((15E)P(r)). We demonstrate that RcaE from Fremyella diplosiphon undergoes the same photocycle and exhibits light-regulated kinase activity. In both RcaE and CcaS, the bilin chromophore is deprotonated as (15Z)P(g) but protonated as (15E)P(r). This change of bilin protonation state is modulated by three key residues that are conserved in green/red CBCRs. We therefore designate the photocycle of green/red CBCRs a "protochromic photocycle," in which the dramatic change from green to red absorption is not induced by initial bilin photoisomerization but by a subsequent change in bilin protonation state.

Loss of cytochrome cM stimulates cyanobacterial heterotrophic growth in the dark.

Although cyanobacteria are photoautotrophs, they have the capability for heterotrophic metabolism that enables them to survive in their natural habitat. However, cyanobacterial species that grow heterotrophically in the dark are rare. It remains largely unknown how cyanobacteria regulate heterotrophic activity. The cyanobacterium Leptolyngbya boryana grows heterotrophically with glucose in the dark. A dark-adapted variant dg5 isolated from the wild type (WT) exhibits enhanced heterotrophic growth in the dark. We sequenced the genomes of dg5 and the WT to identify the mutation(s) of dg5. The WT genome consists of a circular chromosome (6,176,364 bp), a circular plasmid pLBA (77,793 bp) and two linear plasmids pLBX (504,942 bp) and pLBY (44,369 bp). Genome comparison revealed three mutation sites. Phenotype analysis of mutants isolated from the WT by introducing these mutations individually revealed that the relevant mutation is a single adenine insertion causing a frameshift of cytM encoding Cyt c(M). The respiratory oxygen consumption of the cytM-lacking mutant grown in the dark was significantly higher than that of the WT. We isolated a cytM-lacking mutant, ΔcytM, from another cyanobacterium Synechocystis sp. PCC 6803, and ΔcytM grew in the dark with a doubling time of 33 h in contrast to no growth of the WT. The respiratory oxygen consumption of ΔcytM grown in the dark was about 2-fold higher than that of the WT. These results suggest a suppressive role(s) for Cyt cM in regulation of heterotrophic activity.

Green/red light-sensing mechanism in the chromatic acclimation photosensor.

Certain cyanobacteria alter their photosynthetic light absorption between green and red, a phenomenon called complementary chromatic acclimation. The acclimation is regulated by a cyanobacteriochrome-class photosensor that reversibly photoconverts between green-absorbing (Pg) and red-absorbing (Pr) states. Here, we elucidated the structural basis of the green/red photocycle. In the Pg state, the bilin chromophore adopted the extended C15-Z,anti structure within a hydrophobic pocket. Upon photoconversion to the Pr state, the bilin is isomerized to the cyclic C15-E,syn structure, forming a water channel in the pocket. The solvation/desolvation of the bilin causes changes in the protonation state and the stability of π-conjugation at the B ring, leading to a large absorption shift. These results advance our understanding of the enormous spectral diversity of the phytochrome superfamily.

Primary photodynamics of the green/red-absorbing photoswitching regulator of the chromatic adaptation E domain from Fremyella diplosiphon.

Phytochromes are red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the red to far-red light ratio. Cyanobacteriochromes (CBCRs) are distantly related cyanobacterial photosensors with homologous bilin-binding GAF domains, but they exhibit greater spectral diversity. Different CBCR subfamilies have been described, with spectral sensitivity varying across the near-ultraviolet and throughout the visible spectrum, but all known CBCRs utilize photoisomerization of the bilin 15,16-double bond as the primary photochemical event. The first CBCR discovered was RcaE, responsible for tuning light harvesting to the incident color environment (complementary chromatic adaptation) in Fremyella diplosiphon. The green/red RcaE photocycle has recently been described in detail. We now extend this analysis by examining femtosecond photodynamics using ultrafast transient absorption techniques with broadband detection and multicomponent global analysis. Excited-state dynamics in both directions are significantly slower than those recently published for the red/green CBCR NpR6012g4. In the forward reaction, the primary Lumi-G photoproduct arises from the longer-lived excited-state populations, leading to a low photoproduct quantum yield. Using dual-excitation wavelength interleaved pump-probe spectroscopy, we observe multiphasic excited-state dynamics in the forward reaction ((15Z)Pg → (15E)Pr), which we interpret as arising from ground-state inhomogeneity with different tautomers of the PCB chromophore. The reverse reaction ((15E)Pr → (15Z)Pg) is characterized via pump-probe spectroscopy and also exhibits slow excited-state decay dynamics and a low photoproduct yield. These results provide the first description of excited-state dynamics for a green/red CBCR.

Cyanobacteriochrome CcaS regulates phycoerythrin accumulation in Nostoc punctiforme, a group II chromatic adapter.

Responding to green and red light, certain cyanobacteria change the composition of their light-harvesting pigments, phycoerythrin (PE) and phycocyanin (PC). Although this phenomenon-complementary chromatic adaptation-is well known, the green light-sensing mechanism for PE accumulation is unclear. The filamentous cyanobacterium Nostoc punctiforme ATCC 29133 (N. punctiforme) regulates PE synthesis in response to green and red light (group II chromatic adaptation). We disrupted the green/red-perceiving histidine-kinase gene (ccaS) or the cognate response regulator gene (ccaR), which are clustered with several PE and PC genes (cpeC-cpcG2-cpeR1 operon) in N. punctiforme. Under green light, wild-type cells accumulated a significant amount of PE upon induction of cpeC-cpcG2-cpeR1 expression, whereas they accumulated little PE with suppression of cpeC-cpcG2-cpeR1 expression under red light. Under both green and red light, the ccaS mutant constitutively accumulated some PE with constitutively low cpeC-cpcG2-cpeR1 expression, whereas the ccaR mutant accumulated little PE with suppression of cpeC-cpcG2-cpeR1 expression. The results of an electrophoretic mobility shift assay suggest that CcaR binds to the promoter region of cpeC-cpcG2-cpeR1, which contains a conserved direct-repeat motif. Taken together, the results suggest that CcaS phosphorylates CcaR under green light and that phosphorylated CcaR then induces cpeC-cpcG2-cpeR1 expression, leading to PE accumulation. In contrast, CcaS probably represses cpeC-cpcG2-cpeR1 expression by dephosphorylation of CcaR under red light. We also found that the cpeB-cpeA operon is partially regulated by green and red light, suggesting that the green light-induced regulatory protein CpeR1 activates cpeB-cpeA expression together with constitutively induced CpeR2.